Light olefins, ethylene and propylene, serve as feeds for plastics production and for the production of petrochemicals which serve as feeds for plastics production. The demand for light olefins has been steadily increasing and will continue to increase dramatically. Light olefins have traditionally been produced through the process of steam or catalytic cracking. Paraffin dehydrogenation is an alternative source of light olefins. However, the demand for light olefins is outstripping the capacity of traditional sources of light olefins.
The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol and higher alcohols or their derivatives. These alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids and from carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of olefins and other hydrocarbons. Methanol, in particular, is useful in this process which is referred to herein as the methanol-to-olefin (MTO) process.
Molecular sieve catalysts such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. No. 3,928,483; U.S. Pat. No. 4,025,575; U.S. Pat. No. 4,252,479; U.S. Pat. No. 4,496,786; U.S. Pat. No. 4,547,616; U.S. Pat. No. 4,677,243; U.S. Pat. No. 4,843,183; U.S. Pat. No. 4,499,314; U.S. Pat. No. 4,447,669; U.S. Pat. No. 5,095,163; U.S. Pat. No. 5,191,141; U.S. Pat. No. 5,126,308; U.S. Pat. No. 4,973,792; and U.S. Pat. No. 4,861,938.
The MTO process may be generally conducted in the presence of one or more diluents which may be present in the oxygenate-containing feed in an amount between about 1 and about 99 mol-%, based on the total number of moles of all feed and diluent components fed to the reaction zone. Diluents include, but are not limited to, helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water, paraffins, hydrocarbons (such as methane and the like), aromatic compounds, or mixtures thereof. U.S. Pat. No. 4,861,938 and U.S. Pat. No. 4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light olefin products, particularly ethylene.
Generally, the product ratio of ethylene to propylene on a carbon basis varies from about 0.1 to about 10 and, more typically, varies from about 0.8 to about 2.5. Ethylene and propylene are particularly desirable olefins but it has been found that their yields are reduced by the production of medium-weight hydrocarbons such as C4 to C8 olefins, as well as some heavier components. U.S. Pat. No. 5,914,433 proposes cracking medium-weight olefins over a catalyst in vapor phase to increase overall yield of light olefins.
A portion of the medium-weight olefin stream, when cracked, will be converted to paraffinic compounds such as methane, ethane, propane, and heavier hydrocarbons. Unless at least a portion of these compounds are removed, they will build up in the system and reduce the overall efficiency of the process. Therefore, a drag stream comprising C4 and heavier hydrocarbons is removed from the process and used for plant fuel, blended into other hydrocarbon products such as motor gasoline or used as feed to a gasoline alkylation process.
In order to maximize the production of light olefins and to minimize the production of methane produced from cracking the medium-weight olefin stream, diolefins and acetylenes should be minimized in the feed to the olefin cracking zone. Diolefin and acetylene conversion to monoolefin hydrocarbons may be accomplished with a conventional selective hydrogenation process such as disclosed in U.S. Pat. No. 4,695,560.
In a light olefins recovery flow scheme, U.S. Pat. No. 6,486,369 discloses a single selective hydrogenation converter for treating methyl acetylene and propadiene in a feed to a deethanizer column. WO 2004/009519 A1 discloses a fractionation scheme for recovering light olefins from a medium-weight olefin containing stream.